Although current detecting devices having various configurations have been proposed and put into use as current detecting devices, a fluxgate type of current sensor is known as a structurally simple current detecting device that can detect a slight current (for example, refer to PTL 1, which is identified below).
A current sensor of a heretofore known example described in PTL 1 has the structure shown in FIG. 10(a). That is, the current sensor includes circular cores 101 and 102 made of a soft magnetic body, configured to be of the same form and size, an exciting coil 103 wound the same number of times around each of the cores 101 and 102, and a detector coil 104 wound as one so as to encompass each of the cores 101 and 102.
An unshown alternating current power supply is connected to the exciting coil 103, and an unshown detector circuit is connected to the detector coil 104. Further, a measurement target conductor 105, which forms a target for measuring current, is inserted through the centers of the two cores 101 and 102.
The exciting coil 103 is wound around the cores 101 and 102 so that magnetic fields generated in the two cores 101 and 102 are of opposite phases, and cancel each other out when the exciting coil 103 is energized.
Further, temporal variation of a magnetic flux density B generated in each of the cores 101 and 102 when the exciting coil 103 is energized with an exciting current iex is as shown in FIG. 10(b). When a magnetic field size H is within a predetermined range, the magnetic characteristics of the cores 101 and 102 made of a soft magnetic body are such that the magnetic field size H and magnetic flux density B have a linear relationship. However, when the magnetic field size H exceeds a predetermined value, the relationship is such that there is a state of magnetic saturation wherein the magnetic flux density B does not vary, because of which, when the exciting coil 103 is energized with the exciting current iex, the magnetic flux density B generated in each of the cores 101 and 102 changes to a trapezoidal wave form with vertical symmetry, as shown by solid lines in the drawing, and moreover, the phases are in a state displaced 180° from each other.
Herein, assuming that the measurement target conductor 105 is energized by a direct current value I from above, as shown by an arrow, the magnetic flux densities, which correspond to the size of the direct current, overlap, as a result of which the magnetic flux density B is in a state wherein the width of the upper trapezoidal wave of the trapezoidal waves is expanded, while the width of the lower trapezoidal wave is contracted, as shown by broken lines in FIG. 10(b).
Herein, when expressing variation in the magnetic flux density B generated in the two cores 101 and 102 as sinusoidal waves (corresponding to electromotive force), they are as shown in FIG. 10(c). In FIG. 10(c), sinusoidal waves (electromotive force) of a frequency f whose phases are displaced 180° appear, as shown by solid lines in the drawing, corresponding to the trapezoidal waves shown by solid lines in the previously described FIG. 10(b), but as they are displaced by 180°, they cancel each other out. Meanwhile, second harmonics of a double frequency 2f, of the kind shown by broken lines in FIG. 10(c), appear corresponding to the trapezoidal waves shown by broken lines in FIG. 10(b). The phases of the second harmonics are displaced 180°, because of which, when the second harmonics overlap each other, they become a sinusoidal wave signal of the kind shown on the lowest level of FIG. 10(c), and the sinusoidal wave signal is detected by the detector coil 104.
The detection signal picked up by the detector coil 104 corresponds to the current value I of the direct current flowing through the measurement target conductor 105, and it is possible to detect the current value I by processing the detection signal.
Also, a current sensor including one or more first detection transformers that include a primary coil through which a current to be detected is caused to flow and a secondary coil, electrically insulated from the primary coil, magnetically coupled to the primary coil by a magnetic core, and a detection means that includes a means for alternately supplying magnetizing currents of opposite directions to the secondary coil in order to cyclically drive the magnetic core, which includes a means for detecting saturation and causing the direction of a magnetizing current to be inverted accordingly, to a saturated state, and a processing means for outputting an output signal substantially proportional to a detected current, has been proposed as another heretofore known example (for example, refer to PTL 2).
The current sensor further includes a second detection transformer having a low-pass filter, connected to the secondary coil of the first detection transformer, that divides a low frequency or direct current component of a magnetizing current generated in the secondary coil in accordance with a detected current, a primary coil through which a detected current passes, and a secondary coil, wherein the input side of the secondary coil is coupled to an output portion of the low-pass filter, and the output side of the secondary coil is grounded by a resistor in which is generated an output signal of the current sensor.